Tryptophan Fluorescence Quenching in β-Lactam-Interacting Proteins Is Modulated by the Structure of Intermediates and Final Products of the Acylation Reaction

In most bacteria, β-lactam antibiotics inhibit the last cross-linking step of peptidoglycan synthesis by acylation of the active-site Ser of d,d-transpeptidases belonging to the penicillin-binding protein (PBP) family. In mycobacteria, cross-linking is mainly ensured by l,d-transpeptidases (LDTs), which are promising targets for the development of β-lactam-based therapies for multidrug-resistant tuberculosis. For this purpose, fluorescence spectroscopy is used to investigate the efficacy of LDT inactivation by β-lactams but the basis for fluorescence quenching during enzyme acylation remains unknown. In contrast to what has been reported for PBPs, we show here using a model l,d-transpeptidase (Ldt) that fluorescence quenching of Trp residues does not depend upon direct hydrophobic interaction between Trp residues and β-lactams. Rather, Trp fluorescence was quenched by the drug covalently bound to the active-site Cys residue of Ldt. Fluorescence quenching was not quantitatively determined by the size of the drug and was not specific of the thioester link connecting the β-lactam carbonyl to the catalytic Cys as quenching was also observed for acylation of the active-site Ser of β-lactamase BlaC from M. tuberculosis. Fluorescence quenching was extensive for reaction intermediates containing an amine anion and for acylenzymes containing an imine stabilized by mesomeric effect, but not for acylenzymes containing a protonated β-lactam nitrogen. Together, these results indicate that the extent of fluorescence quenching is determined by the status of the β-lactam nitrogen. Thus, fluorescence kinetics can provide information not only on the efficacy of enzyme inactivation but also on the structure of the covalent adducts responsible for enzyme inactivation

Design and synthesis of iminosugar-based inhibitors of glucosylceramide synthase: the search for new therapeutic agents against Gaucher disease

A series of iminosugars bearing two or three alkyl chains ('iminoglycolipids') were designed as ceramide mimics and analogues of N-butyl 1-deoxynojirimycin (N-Bu DNJ, Zavesca®). This orally active iminosugar inhibits the biosynthesis of glucosylceramides, which accumulate pathologically in macrophages of patients with Gaucher disease (substrate reduction therapy, SRT). Molecular modeling and kinetic experiments have suggested that N-Bu DNJ is a competitive inhibitor that mimics the ceramide acceptor but not the donor substrate (UDP-glucose) in the glucosylceramide synthase-catalyzed process. Kinetic measurements were made with the glucosyltransferase to assess the selectivity of the new iminoglycolipids with respect to the length (C4 or C8) and the position of the second alkyl chain (C-1, O-2 and/or O-4). This structure-activity relationship study showed that the addition of a second alkyl chain, to obtain better ceramide mimics, led to less potent inhibitors. Moreover, the synthase active site did not discriminate inhibitors differing by the position of the second alkyl chain (C-1, O-2 or O-4). Best inhibition was found for 1,5-dideoxy-1,5-imino-N-octyl-4-O-octyl-d-glucitol (IC50 134 μM). © 2005 Elsevier Ltd. All rights reserved

Chronic endocarditis due to Legionella anisa: a first case difficult to diagnose

Endocarditis due to Legionella spp. is uncommon but presumably underestimated given the prevalence of Legionellae in the environment. We report a first and unusual case of chronic native valve endocarditis due to L. anisa and advocate that the diagnosis of endocarditis be made collaboratively between the cardiologist, surgeon, microbiologist and pathologist